Rotator cuff muscles are commonly thought of as stabilizers of the glenohumeral joint during shoulder movements. Recent evidence however, indicates only a marginal contribution of the rotator cuff to joint stability.

This study investigated shoulder muscle activation patterns in fourteen healthy subjects during dynamic external rotation performed at 90° of abduction. An adjustable sling was used to modify the support of the upper arm – three conditions were examined: with the upper arm 100% supported, 50% supported and unsupported. Participants performed full range rotation exercises while standing; the exercises were randomized by support load conditions.

The infraspinatus had the highest activation levels over the three tasks, nevertheless the activation levels did not increase as support decreased. The supraspinatus muscle on the other hand did show increased activation as the upper arm was less supported. This indicates the infraspinatus’ activity is (as expected during external rotation) predominantly aimed a producing movement, while the supraspinatus indeed functions as a stabilizer.

Subscapularis activation levels remained relatively low over all conditions: it has only a marginal role in stabilizing the glenohumeral joint during external rotation. Deltoid muscle activation showed similar patterns to the supraspinatus, suggesting a similar role for both muscles. > From: Tardo et al., Clin Anat 26 (2013) 236-243. All rights reserved to Wiley Periodicals, Inc.

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